The proposal seeks to understand the mechanisms by which cytotoxic T lymphocytes (CTL) recognize and kill target cells. The work is based upon the availability of large quantities of purified human histocompatibility antigens and an in vitro system in which mouse CTL precursors are induced to differentiate upon being presented with the antigen reconstituted into phospholipid vesicles. This induction is antigen-specific, and the CTL formed specifically recognize the appropriate antigens on the target cell surface. A binding assay will be developed in which CTL interacts radiolabelled histocompatibility antigens in phospholipid vesicles. This will allow characterization of the binding. Antigen-containing vesicles will also be used in an attempt to inhibit the target cell lysis. The proposal also will examine the cytotoxic mechanism by using histocompatibility antigen-containing vesicles or erythrocyte ghosts to which these vesicles have been fused, as model targets in order to assess the influence of target cell membrane composition and organization on susceptibility to lysis. In addition, by the use of internal markers of various sizes, the size of the initial lesion will be determined. The availability of the present system, and the considerable precedent for such an approach, suggest the feasibility of using immobilized histocompatibility antigens as immunoadsorbents for the purification of antigen binding molecules from T cells (receptors). By using biosynthetically labeled T cells, attempts will be made to purify and characterize such molecules. Finally, the production of antisera specific for differentiated CTL and the use of cell monolayers as adsorbents are contemplated as a means for the isolation of molecules unique to differentiated lymphocytes.